Method, alloy and component

ABSTRACT

A method for heat treating a nickel base alloy includes the steps of: a. heating a nickel base alloy to at least its delta (δ) phase solvus temperature, and lower than its incipient melting temperature for a predetermined time sufficient to dissolve,substantially all of the nickel base alloy&#39;s delta (δ) phase, and b. cooling the nickel base alloy to a temperature below the gamma prime (ν) precipitation temperature at a rate sufficient to precipitate the alloy&#39;s chromium carbide and gamma prime (ν) in a serrated grain boundary.

BACKGROUND AND SUMMARY

The present invention concerns a method for heat treating a nickel basealloy. The present invention also concerns a nickel base alloy and acomponent comprising the nickel base alloy.

It should be noted that the expression “nickel base alloy”, as usedthroughout this document, is intended to mean any alloy whose mainconstituent is nickel and the expression includes nickel basesuperalloys or high performance alloys.

Nickel base alloys are used in industry because of their ability towithstand a variety of severe operating conditions involving corrosiveenvironments, high temperatures, high stresses, and combinationsthereof. The mechanical properties of a nickel base alloy are closelyrelated to the microstructure of the alloy, which is, in turn,controlled by the processing and heat treatment to which the alloy issubjected.

Precipitation hardening is a heat treatment that relies on changes in analloy's solid solubility with temperature to produce fine particles ofan impurity phase, which impede the movement of dislocations or defectsin the alloy's crystal lattice. Since dislocations are often thedominant carriers of plasticity (deformations of an alloy under stress),this serves to harden the alloy. In precipitation-hardened nickel basealloys, there are two principal strengthening phases: namely gamma prime(v1) phase precipitates and gamma double prime (γM) phase precipitates.Both the gamma prime and gamma double prime phase are stoichiometric,nickel-rich intermetallic compounds. However, the gamma prime phasetypically comprises aluminum and titanium as the major alloyingelements, i.e., Ni3(Al, Ti); while the gamma double prime phase containsprimarily niobium, i.e., Ni3Nb.

The precipitation hardening process for a nickel base alloy generallyinvolves solution treating the alloy by heating it at a temperaturesufficient to dissolve substantially all of the gamma prime phase andgamma double prime phase precipitates that exist in the alloy (i.e. at atemperature near, at, or above the solvus temperature of theprecipitates), cooling the alloy from the solution treating temperature,and subsequently aging the alloy in one or more aging steps. Aging isconducted at temperatures below the solvus temperature of the gammaprecipitates in order to permit the desired precipitates to develop in acontrolled manner.

For example, the recommended heat treatment for alloy Alvac 718Plus™, (anickel base alloy developed at ATI Allvac an Allegheny TechnologiesCompany, Monroe, US) involves solution treating the alloy at atemperature of 954° C. for one hour, water cooling the alloy and agingthe alloy in a two-step aging process. The first aging step involvesheating the alloy at a first aging temperature of 788° C. for 8 hours.The alloy is then air cooled to a second aging temperature of 704° C.and aged at a second aging temperature for 8 hours. The alloy is thenair cooled to room/ambient temperature.

Alvac 718Plus™ has mechanical properties that are better than themechanical properties of alloy 718 (a nickel base alloy comprisingchromium and iron, which is strengthened by niobium and/or aluminiumand/or titanium) and that are comparable to the mechanical properties ofWaspaloy® at temperatures up to 704° C. The improved high temperaturemechanical properties of Alvac 718Plus™ are due to the high fraction andhigh stability of the main strengthening gamma prime phase in comparisonto the gamma double prime phase in alloy 718. The high amount ofaluminium and titanium in Alvac 718Plus™ namely increases the thermalstability of the alloy and the high amount of niobium increases thestrengthening effect of the gamma prime phase significantly and makesits precipitation much more sluggish which will consequently enhance thehot workability and weldability of the alloy.

US patent application publication no. 2005/0072500 concerns methods ofheat treating nickel base alloys, and in particular 718-type nickel basealloys, to develop a desired microstructure that can impart thermallystable mechanical properties. This document discloses that theprecipitation of a controlled amount of delta phase precipitates canstrengthen a nickel base alloy's grain boundaries. Delta phaseprecipitates have the same composition as gamma double prime phaseprecipitates (i.e., Ni3Nb) and are formed at temperatures higher than649° C. in 718-type alloys, at which temperature the gamma double primephase becomes unstable and transforms into the more stable 5 phase (or“delta phase”), The document also discloses that the precipitation of acontrolled amount of delta phase precipitates contributes to reducednotch sensitivity (whereby notch sensitivity is a measure of thereduction in strength of a metal caused by the presence of a stressconcentrator, for example .a surface inhomogeneity such as a notch,thread, hole, sudden change in section, crack, or scratch) and improvedstress rupture life and ductility in the alloy at elevated temperatures.For these reasons, US patent application publication no. 2005/0072500states that it is advantageous to retain delta phase precipitates in anickel base alloy.

It is desirable to provide a method for heat treating a nickel basealloy.

A method according to an aspect of the present invention comprises thesteps of; a) heating a nickel base alloy to at least its delta (δ) phasesolvus temperature, and lower than its incipient melting temperature fora predetermined time sufficient to dissolve substantially all of thenickel base alloy's delta (δ) phase, and b) slow cooling the nickel basealloy to a temperature below the gamma prime (Y) precipitationtemperature at a rate sufficient to precipitate the alloy's chromiumcarbide and gamma prime (γ1) in a serrated grain boundary. Such a methodhas been found to improve the creep resistance and/or the crackpropagation resistance of a nickel base alloy.

The expression “substantially all” means that the microstructure of thenickel base alloy will exhibit at least one serrated grain boundary, ora plurality or majority of serrated grain boundaries, after it has beensubjected to a method according to the present invention.

As regards the expression “for a predetermined time,” it will beappreciated by those skilled in the art that the exact treatment timerequired to dissolve substantially all of the delta phase will depend onseveral factors, including, but not limited to the size of the workpiece comprising nickel base alloy that is being treated. The bigger thework piece, the longer the holding time required to achieve the desiredresult will be.

Conventional high temperature heat treatment methods, such as the heattreatment recommended for Allvac 718Plus™, in which an alloy is heatedabove the gamma prime solvus temperature, result in the formation ofplaner grain boundaries. Serrated boundaries can be formed by slowcooling a nickel base alloy through its gamma prime precipitation rangeat the rate specified in step b) of the method according to the presentinvention. Serrations develop when the gamma prime phase precipitatesheterogeneously in the form of rod-shaped particles on migrating grainboundaries and allows unpinned grain boundary segments to fill the spacebetween them. Without wishing to be bound by any particular theory,these serrations are believed to impede grain-boundary sliding and toforce deformation to occur more uniformly through grain interiors andgrain-boundary regions. The initiation of cracks due to localizedgrain-boundary deformation is therefore inhibited. These serrations arealso believed to impede crack propagation along the grain boundaries.

In other words, the method according to the present inventionstrengthens a nickel base alloy's grain boundaries by “locking” or“pinning” the grain boundaries in place, which results in the creationof a nickel base alloy that exhibits significantly improved creepproperties such as improved creep resistance and/or crack propagation.

Experiments were performed to compare the notch sensitivity of nickelbase alloys that had been subjected to the method according to thepresent invention with the notch sensitivity of commercially availablenickel base alloys that had been heat treated in accordance withmanufacturers' recommendations by conducting Time-for-Rupture NotchTension Tests (in accordance with the American Society for Testing andMaterials' ASTM Standard E292). These tests involved the determinationof the time for rupture of notched nickel base alloy specimens underconditions of constant load and temperature.

The notch sensitivity of nickel base alloys that had been subjected tothe method according to the present invention was found to be much lowerthan the notch sensitivity of commercially available nickel base alloys,meaning that the method according to the present invention results inthe production of more ductile nickel base alloys.

According to an embodiment of the invention, in step b) the nickel basealloy is cooled at a rate of 1.0° C. per minute or less, 0.7° C. perminute or less, or 0.5° C. per minute or less, and at a rate 0.05° C.per minute or higher, or 0.1° C. per minute or higher.

According to an embodiment of the invention the method comprises thestep of c) heating the nickel based alloy at a first aging temperaturebelow the solvus temperatures for the gamma prime phase and the gammadouble prime phase to form primary precipitates of the gamma prime andgamma double prime phase and, optionally, the method comprises the stepof d) heating the alloy at a second aging temperature below the solvustemperatures for the gamma prime phase and the gamma double prime phaseto form finer secondary precipitates of the gamma prime and gamma doubleprime phase (since the presence of gamma prime and gamma double primephase precipitates having a distribution of sizes, as opposed to auniform precipitate size is believed to improve the mechanicalproperties of the nickel base alloy).

According to an embodiment of the invention the method according to thepresent invention may be carried out on a nickel base alloy that has thefollowing composition in weight-%: Cr 17-21, C 0.01-0.05, Mn max 0.35,Si max 0.35, P 0.004-0.020, S max 0.0025, Mo 02.5-3.1, Nb 5.2-5.8, Ti0.5-1, Al 1.2-1.7, Co 8-10, W 0.8-1.4, B 0.003- 0.008, Cu max 0.3, Fe8-10, and balance Ni and normally occurring impurities.

According to another embodiment of the invention the method according tothe present invention may be carried out on a 718-type nickel basealloy, such as Allvac 718Plus™.

When the nickel base alloy is Allvac 718Plus™ or when it has thecomposition specified above, the nickel base alloy is preferably heatedto a temperature that is sufficient to dissolve substantially all of thenickel base alloy's delta (δ) phase but that does not exceed atemperature that will promote grain growth, namely to a temperature of1040° C. ±14° C. According to an embodiment of the invention the nickelbase alloy is held at that temperature until substantially all of thenickel base alloy's delta (δ) phase has been dissolved, preferably for0.5 hour in step a). According to another embodiment of the inventionthe alloy is then slow cooled from 1040° C. to a temperature of 650°C.±14° C. or lower in step b) since all of the alloy's chromium carbideand gamma prime (Y) should theoretically have precipitated in a serratedgrain boundary once the alloy has been cooled to 650° C. According to afurther embodiment of the invention the nickel base alloy maysubsequently be aged by heating the nickel base alloy to a first agingtemperature of 788° C.±14° C. in step c). According to an embodiment ofthe invention the nickel base alloy is held at that temperature forabout 8 hours in step c) i.e. for 8 hours or more. According to anotherembodiment of the invention the nickel base alloy is then cooled from788° C.±14° C. to a second aging temperature of 704° C.±14° C. in stepd). According to a further embodiment of the invention the nickel basealloy is held at that temperature for about 8 hours in step d) i.e. for8 hours or more.

It should be noted that the first and second aging temperatures andholding times cited above in steps c) and d) are specific to Allvac718Plus™ or a nickel base alloy that has the composition specifiedabove. When heat treating a different nickel base alloy, the agingtemperatures and holding times recommended by manufacturers should beused in steps c) and d).

The method according to the present invention may be used in conjunctionwith of a variety of nickel base alloy compositions such as any nickelbase alloy that is strengthened by gamma prime and gamma double prime.

The present invention also concerns a nickel base alloy at least part,or the whole, of which has been subjected to a method according to anyof the embodiments of the invention. The nickel base alloy may be a castor wrought nickel base alloy in the form a billet, coil, sheet, bar,ingot, rod, tube or any other desired form.

According to an embodiment of the invention the microstructure of thenickel base alloy exhibits chromium carbide and gamma prime (Y)precipitated in a serrated grain boundary.

The present invention also concerns a component comprising a nickel basealloy according to any of the embodiments of the invention. Thecomponent may for example be a turbine or compressor disc, blade, rotor,case, shaft, spacer, seal or fastener or any other component used in anyapplication in which it may be subjected to operating conditionsinvolving corrosive environments, high temperatures and/or highstresses.

The inventive nickel base alloy and component are intended for useparticularly, but not exclusively in aircraft and industrial gasturbines, steam turbine power plants, submarines, rocket engines,turbochargers and valves in reciprocating engines, heat treatingequipment, chemical processing equipment, gasification and liquefactionsystems, in marine applications and components in pulp and paper mills.

The present invention further concerns an aircraft engine comprising acomponent according to any of the embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be further explained by means ofnon-limiting examples with reference to the appended figures where;

FIG. 1 schematically shows a temperature/time diagram depicting a methodaccording to an embodiment of the invention,

FIG. 2 shows the test sample geometry used for testing the notchsensitivity of a nickel base alloy,

FIG. 3 shows a micrograph of the microstructure of a nickel base alloyafter it has been subjected to a method according to an embodiment ofthe invention, and a schematic representation of the grain boundariesshown in the micrograph, and

FIG. 4 schematically shows an aero-engine comprising componentsaccording to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically shows a method for heat treating at least part ofcommercially available Allvac 718Plus™ (that has an incipient meltingtemperature of 1260° C.). The method comprises the steps of;

a) heating at least one part of the alloy to its delta (δ) phase solvustemperature, 1040° C., and holding it at that held at that temperaturefor 0.5 hour to dissolve substantially all of the delta (δ) phasepresent in the alloy's as-received microstructure ,b) slow cooling the at least one part of the alloy from the solutiontemperature of step a) to 650° C. at a rate of 0.5° C. per minute orless, without interruption, to precipitate the alloy's chromium carbideand gamma prime (γ1) in a serrated grain boundary (step b), the alloymay then be cooled to room temperature,c) heating the at least one part of the alloy to a temperature of 788°C. and holding it at that temperature for about 8 hours, andd) cooling the at least one part of the nickel base alloy from 788° C.to 704° C. and holding it at that temperature for about 8 hours.

The at one least part of the nickel base alloy that has been subjectedto such a method will have a microstructure that is entirely free, or atleast substantially free of delta phase, wherein gamma prime phaseprecipitates are the predominant strengthening precipitates. Moreparticularly, such a method will produce a nickel base alloy that hasimproved creep resistance and/or crack propagation resistance.

FIG. 2 shows test sample 10 geometry that was used to conductTime-for-Rupture Notch Tension Tests to compare the notch sensitivity ofnickel base alloys that had been subjected to the method according tothe present invention with the notch sensitivity of commerciallyavailable nickel base alloys that had been heat treated in accordancewith manufacturer's recommendations.

Test samples 10 were manufactured from a 3 mm thick sheet of Allvac718Plus™ using electric discharge wire (EDW) machining and tested inaccordance with the ASTM 292 Standard. Test samples 10 were tested in athermally insulated induction furnace in an air environment at atemperature of 704° C. using a load of 690 MPa. To avoid temperaturefluctuations while the tests were taking place, all the tests began bystabilising the temperature. Six thermocouples were used for controllingthe temperature of the induction furnace in which the test samples 10were placed for testing. Three thermocouples were attached to the testsample 10, one at the centre of the test sample and one at each endthereof, and three thermocouples were evenly spaced and embedded in thedoor of the induction furnace. The thermocouples were connected to atemperature regulator and to a computer which logged the temperature andtime.

It was found that Allvac 718Plus™ that had been subjected to the methodshown schematically in FIG. 1 ruptured after 25 minutes whereascommercially available Allvac 718Plus™ that had been subjected to therecommended heat treatment (as outlined in the Background of theInvention above) ruptured after 13 minutes.

This significant improvement in creep properties is believed to be dueto the serrated grain boundary that is formed in Allvac 718Plus™ when ithas been subjected to a method according to the present invention. FIG.3 shows a micrograph showing the microstructure of a nickel base alloyafter it has been subjected to a method according to an embodiment ofthe invention and a schematic representation of the grain boundariesshown in the micrograph. The microstructure shown in FIG. 3 exhibitschromium carbide and gamma prime (Y) precipitated in serrated grainboundaries that resemble the toothed edge of a saw, the presence ofserrated grain boundaries indicating that the Allvac 718Plus™ has beensubjected to a method according to the present invention. The arrow inFIG. 3 indicates one of the most clearly visible serrated grainboundaries 11 in the micrograph.

FIG. 4 schematically shows a cross section through an aero-engine 12which comprises a plurality of components, namely a diffuser case 13, alow pressure turbine (LPT) case 14, vanes 15 and a turbine exhaust case16, comprising a nickel base alloy according to an embodiment of theinvention. It should be noted that an entire component made of a nickelbase alloy need not necessarily be subjected to a method according to anembodiment of the invention, it may be sufficient to heat treat one ormore parts of the component that is/are subjected to the highesttemperatures and/or highest stresses 15 when the aero engine is in use.

Further modifications of the invention within the scope of the claimswill be apparent to a skilled person.

1. Method for heat treating a nickel base alloy, comprising the stepsof: a) heating a nickel base alloy to at least its delta (δ) phasesolvus temperature, and lower than its incipient melting temperature fora predetermined time sufficient to dissolve substantially all of thenickel base alloy's delta (δ) phase, and b) cooling the nickel basealloy to a temperature below the gamma prime (γ1) precipitationtemperature at a rate sufficient to precipitate the alloy's chromiumcarbide and gamma prime (γ1) in a serrated grain boundary.
 2. Methodaccording to claim 1, wherein the nickel base alloy is cooled at a rateof 1.0° C. per minute or less in step b).
 3. Method according to claim1, wherein the nickel base alloy is cooled at a rate of 0.7° C. perminute or less in step b).
 4. Method according to claim 1, wherein thenickel base alloy is cooled at a rate of 0.5° C. per minute or less instep b).
 5. Method according to claim 1, wherein the nickel base alloyis cooled at a rate of 0.05° C. per minute or higher in step b). 6.Method according to claim 1, wherein the nickel base alloy is cooled ata rate of 0.1° C. per minute or higher in step b).
 7. Method accordingto claim 1, wherein the method further comprises the step of: c) heatingthe nickel based alloy at a first aging temperature below the solvustemperatures for the gamma prime phase and the gamma double prime phaseto form primary precipitates of the gamma prime and gamma double primephase.
 8. Method according to claim 1, wherein the method furthercomprises the step of: d) heating the alloy at a second agingtemperature below the solvus temperatures for the gamma prime phase andthe gamma double prime phase to form secondary precipitates of the gammaprime and gamma double prime phase.
 9. Method according to claim 1,wherein the nickel base alloy has the following composition in weight-%:Cr 17-21 C 0.01-0.05 Mn max 0.35 Si max 0.35 P 0.004-0.020 S max 0.0025Mo 02.5-3.1 Nb 5.2-5.8 Ti 0.5-1 Al 1.2-1.7 Co 8-10 W 0.8-1.4 B0.003-0.008 Cu max 0.3 Fe 8-10 balance Ni and normally occurringimpurities.
 10. Method according to claim 1, wherein the nickel basealloy is a 718-type nickel base alloy.
 11. Method according to claim 10,wherein the nickel base alloy is Alivac 718PlUS™.
 12. Method accordingto claim 9, comprising the step of heating the nickel base alloy to atemperature of 1040° C.±14° C. in step a).
 13. A method according toclaim 1, comprising the step of holding the nickel base alloy at theheated temperature for 0.4 to 0.8 hour in step a).
 14. A methodaccording to claim 1, comprising the step of cooling the nickel basealloy to a temperature of 650° C.±14° C. in step b).
 15. A methodaccording to claim 7, comprising the step of heating the nickel basealloy to a temperature of 788° C.±14° C. in step c).
 16. A methodaccording to claim 7, comprising the step of holding the nickel basealloy at the first aging temperature for about 8 hours in step c).
 17. Amethod according to claim 8, comprising the step of cooling the nickelbase alloy to a temperature of 704° C.±14° C. in step d).
 18. A methodaccording to claim 8, comprising the step of holding the nickel basealloy at the first aging temperature for about 8 hours in step d). 19.Nickel base alloy, wherein at least part of the nickel base alloy hasbeen subjected to a method according to claim
 1. 20. Nickel base alloyaccording to claim 19, microstructure exhibits chromium carbide andgamma prime (Y) precipitated in a serrated grain boundary. 21.Component, comprising a nickel base alloy according to claim
 19. 22.Component according to claim 21, wherein is a turbine or compressordisc, blade, case, shaft or fastener.
 23. Aircraft engine comprising acomponent according to claim 21.